We have identified a population of cells in murine bone marrow which we designated as pre-T cells because, although they lack cell surface antigens expressed by peripheral T and B lymphocytes, they respond to phytohemagglutinin by proliferation and the expression of Thy-1+, Lyt-2+ phenotype. They are present in the marrow of nude and thymectomized mice. On shallow Ficoll gradients all bone marrow pre-T cells can be concentrated in one cell fraction which facilitates their analysis. Studies with long-term parabiosis between CBA/H and CBA/H T6T6 mice indicate that unlike recirculating T cells in the spleen and CFU-S in the bone marrow, there is no mixing of pre-T cells in the marrow of the two parabionts suggesting long-term self-maintenance of their precursors within the marrow. The bone marrow cells that augment the tumor neutralization capacity of secondarily in vitro sensitized TSTA-specific T cells sediment in a different fraction than pre-T cells, indicating that pre-T cells do not directly participate in cell interaction between networks involved in the neutralization of methylcholanthrene-induced sarcomas. Multispecific progenitors of cytotoxic T cell precursors (Pre-CLP) can, however, be demonstrated in the same cell fraction as pre-T cells after the fraction has been depleted of T cells. The relationship of the two cell types is being investigated, and the specificity repertoire of Pre-CLP recovered from the bone marrow is being compared with those found in the periphery and the thymus. The identity of bone marrow-derived cells that collaborate with TSTA-specific T cells in tumor neutralization is being pursued on the basis of a working hypothesis that the interaction takes place in context of a delayed-type hypersensitivity reaction against tumor antigens.